This article introduces the non-ideal behavior of high-frequency inductors and helps you select the right capacitors and inductors for applications such as matching networks, DC blocks, crystals, and power supply decoupling. In addition to inductance, inductors also contain parasitic capacitance and parasitic resistance. The figure below describes a model of a real inductor.
Rdc is the ohmic resistance because the conductivity of the inductor is finite. Rac is the frequency-dependent resistance representing the inductor core losses. The parasitic capacitance is the result of the capacitance between the inductor turns. Rac is very high at low frequencies and is usually ignored. The effective impedance of the inductor is
Parasitic capacitance has very high impedance at low frequencies and has little effect on the overall impedance because it is parallel to the inductor. As frequency increases, the impedance created by the capacitance (XCp) decreases, and the impedance created by the inductor increases (XL). XL and XCp eventually become equal at a certain frequency. This frequency is the self-resonant frequency (SRF) of the inductor. Since Rdc is usually very low, the inductor behaves as an open circuit or high impedance at this frequency.
The inductor used in the matching network (the inductance value is very important) should have an SRF well above the operating frequency. When the inductor is used for power supply filtering, it is wise to choose an inductance value with an SRF close to the noise frequency.
Quality Factor
The quality factor (Q) of an inductor (L) is the ratio of the reactance of the inductor to its resistance (R) at a given frequency (f).
It is important to ensure that the Q factor is high for the operating frequency used in the matching network. An inductor with a low Q value has a large impedance. When using low Q components, even a poor impedance match in the matching network can mislead S11 because most of the energy is not delivered to the load, but is wasted as heat in the resistor.
Inductor Recommendations
1. For matching networks, use only high-Q inductors with an SRF much higher than the operating frequency.
2. For power supply filtering, use an inductor with an SRF close to the noise frequency.
3. Do not place inductors in parallel and close to each other. The mutual inductance between them causes crosstalk. Make the inductors or unrelated parts orthogonal to each other.
4. RF ceramic inductors are cost-effective and have high SRF, but have lower Q and current capacity, especially for higher value inductors.
5. Wirewound inductors have lower DC resistance and therefore higher Q and current capacity. For high value inductors, wirewound inductors are preferred over ceramic inductors.
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